This invention generally relates to the microfinishing of workpieces. More particularly, the invention relates to an apparatus and method for microfinishing and truing the interior cylindrical surface of a workpiece such as an engine cylinder liner.
It is common for the combustion cylinders of an internal combustion engine, and in particular large diesel engines, to be fitted with removable cylinder liners. These liners are positioned within the engine block so that when damage or wear becomes apparent, the liner can be easily and relatively inexpensively replaced. Without a removable cylinder liner, the damaged cylinder would require resizing of the pistons, reboring or replacement of the engine block, all of which are expensive repair procedures.
The cylinder liner itself is a seamless tube having a cylindrical interior surface whose diameter closely corresponds to that of the piston. The interior cylindrical surface of the liner must exhibit a diameter within close tolerances of the piston diameter and also must have precise microfinish characteristics. Since the direct forming of cylinder liners within such tolerances has; proven difficult, the interior cylindrical surfaces have typically been machined to a rough diameter and then are microfinished by apparatuses such as stone honing machines.
Stone honing machines used for machining and truing interior cylindrical surfaces include a tool that is adapted to be axially moved while it is rotated about a central axis within the liner. The abrasive stones are spaced circumferentially around the tool so that their finishing surfaces face radially outward. When the tool is rotated, the stones are biased radially outward so that the stones abrade the inner cylindrical surface. By axially moving the tool through the cylinder, the entire inner surface is machined.
While the tools work effectively, the stones themselves have exhibited a tendency to crack or chip and therefore must be frequently replaced and require a significant amount of tool maintenance. Also, the stones experience form changes due to uneven wear caused by the uneven surface of the workpiece itself. Since the uneven wear of the stones will not enable the stones to provide a uniform machining action, subsequently machined parts will contain errors or defects. Additional limitations of stone honing machines include the considerable expense of the stone hones and the limited abrasive performance of the stone hones themselves. Moreover, since the abrasive tools are typically compliantly mounted, they tend to follow any geometric imperfections in the cylinder bore as opposed to correcting them.
As described in U.S. Pat. No. 4,991,361, an apparatus used for machining and truing the inner cylindrical surfaces of a tube uses abrasively coated film material over pressure members which force the film to abrade the surface. When in use, the tool, including the film material which is carried on a spool within the tool, is axially moved while being rotated.
While honing apparatuses using abrasive film material according to the above referenced patent appears workable, it is believed to have certain shortcomings. The machining rate of such an apparatus would be limited because, over the course of machining, only a small total area of film material is actually in contact with the workpiece resulting in a low rate of material removal. A further limitation in the above referenced apparatus is that the film material is stored in the tool which limits the amount of film which can be stored, necessitating frequent refilling of the spools.
It is therefore desirable to provide an apparatus which achieves a high rate of material removal from the interior cylindrical surface of a workpiece while producing an extremely consistent smooth surface with the special finish required by engine manufacturers to reduce engine emission output. It is an objective of the present invention to provide a microfinishing apparatus which achieves these desires.
It is another object of this invention to provide a microfinishing apparatus in which a large supply of abrasive film material can be made available for use thereby decreasing the frequency witch which the supply roll must be changed.
Another objective is to provide a cylinder microfinishing apparatus which is capable of exerting an increased amount of working force on the cylindrical surface for more effective machining. This is achieved in part by spacing apart a number of working areas mounted to a rigid body tool element engaging the workpiece. The use of rigid body tool elements further aids in providing correction of geometry imperfections in the workpiece.
The tool according to this invention can be used in various manners. In one embodiment, the machine includes a headstock which receives and holds the workpiece without obstructing access to the interior cylindrical surface. Once secured within the headstock, the workpiece is rotated about a working axis which is generally coaxial with the axis defined by the rough interior cylindrical surface.
The microfinishing tool of this invention has first and second halves or shoes that are themselves rigid and capable of being outwardly forced apart from one another. Each half of the tool defines pressure surfaces located at circumferentially spaced positions which press the abrasive coated film against the workpiece. By circumferentially spacing the pressure surfaces of the tools, the forces exerted against the workpiece surface are greater than the force separating the tool halves.
A supply of abrasive film material is remotely located relative to the tool and is mounted so that the film material wraps over the tool axially. The film material axially extends over the pressure surfaces during microfinishing to increase the percentage of film material in contact with the cylinder liner during microfinishing and enables the supply of film to be remote from the tool.
In accordance with the invention, the tool is positioned within the liner and the two shoes of the tool are biased radially outward causing the abrasive film material to contact the interior cylindrical surface of the liner. Relative axial and rotational movement between the tool and workpiece occurs so that the entire cylindrical surface of the workpiece is microfinished and a desired crosshatched microgroove pattern is formed in the surface. The abrasive film material is provided so that a single strip of film material extends axially over one pressure area on each of the tools, effectively enabling a single piece of the abrasive film material to contact the workpiece along two diametrically opposing strips. This approach reduces the number of supply rolls which are needed, enables more efficient use of the abrasive film material, and ensures a high rate of material removal.
The novel tool according to this invention can be used with various machine tool configurations. For example, the liner's longitudinal axis can be vertical or horizontal during machining. Configurations in which the tool rotates, and in which the workpiece is held stationary or vice versa are possible, or both elements can be moved together. If desired, oscillating motions as opposed to complete rotations of the tool or workpiece could be implemented.
Additional, benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.